Effects of altitude on compression readings

Does altitude effect compression readings? As you go higher in altitude the air is thinner so does the PSI numbers in a compression reading slso go down? If an engine is producing lets say 150 psi compression at sea level then at 5,000 ft what will the reading be?

Yes it will be lower, your FSM should have a chart for your motor compression readings at different altitudes.
If you wait someone will post it for you most likely.

The problem is the question you are asking is like shooting a moving target :laugh: I can tell you that at standard temperature and pressure (29.92 in/hg @ 15 deg/Celsius), you generally loose close to an inch of mercury for every 1000 ft of elevation gain.

Each inch of Mercury is equal to approximately .5 psi (29.92"hg = 14.7 psi) So in your case, 5000' ele. would equate to 2.5 psi. The only issue with all this info is that atmospheric conditions are RARELY at standard temp and pressure

Well don't think 2.5 psi drop in compression is right for 5000ft. Engines lose 25-30% of power here in Colorado, I'm at 6200ft. I think my 2 J's set close r to 120-125 cranking pressure. My friends 850GS was only 100, when I first checked it cold and not much more hot. He was told he needed a rebuild by 1 mechanic. His FSM had a chart and it was spot on for this high.

porchev914 wrote:

The problem is the question you are asking is like shooting a moving target :laugh: I can tell you that at standard temperature and pressure (29.92 in/hg @ 15 deg/Celsius), you generally loose close to an inch of mercury for every 1000 ft of elevation gain.

That means roughly 3.3% loss for each 1000feet, about 20% down at 6000 feet. If the standard compression is 160psi, then I would guess about 125 psi at 6000 feet?

porchev914 wrote:

Each inch of Mercury is equal to approximately .5 psi (29.92"hg = 14.7 psi) So in your case, 5000' ele. would equate to 2.5 psi.

But that is a loss of 2.5 psi of atmospheric pressure to fill the cylinder which does not equate to 2.5 psi less compression.

Motor Head wrote:

Well don't think 2.5 psi drop in compression is right for 5000ft.

Well, it may not be. I only stated atmospheric pressure drop, not compression drop ,but my data is correct Bountyhunter took my data 1 step further and broke it down into percentages, which looks correct.